I think for n00bies IE is great, Microsoft's support is excellent. But for those of us that can maintain our own computers, IE is far too lacking in features for heavy use.
I think the issue of open source is a none issue. PHP is open source and is very successful.
Then again it all goes back to what suits the person.
The fact is, Firefox isn't that buggy. I use it for hours a day, every day, and I am yet to find any significant problems